Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at a high pressure.
In an article by Perez, V. entitled “The 4.5 MMTBA LNG Train—A Cost Effective Design”, published on 4 May 1998 from the International Conference and Exhibition on Liquefied Natural Gas, there is discussed plant designs for attaining a 4.5 MMTPA nominal capacity LNG train. This involves a set of heat exchangers for cooling a treated feed gas before entering a scrub column. The overhead vapour from the scrub column is then liquefied in a Main Cryogenic Heat Exchanger (MCHE) against a mixed refrigerant. The article states that the propane refrigerant system utilises propane evaporating at four pressure levels, the heat exchangers for which are shown in FIG. 1 as being separate from those for the feed gas. FIG. 3 shows a Frame 7 gas turbine driving a propane compressor and High Pressure Mixed Refrigerant (HPMR) compressor, and the four propane heat exchangers.